Anisotropic resin-bonded magnets, made of magnet powders embedded in resin matrix, are mainly formed by compression molding. In compression molding, the compound which is mixture of magnet powder and resin is filled into a mold and compressed into the desired shape. During the whole compression molding process, the compound is kept in a magnetic field of 8-12 kOe to align the magnetization direction of magnet powder to the direction of magnetic field by letting the magnet powder particles to rotate and displace.
Concerning the compression molding method for the anisotropic resin-bonded magnets, there are two notable Patents that disclose the methods to improve the alignment in the magnet by using rubber mold to provide hydrostatic pressure on the compound. One is Japanese Patent Application Laid-Open (kokai) No. 7-130566. In the invention the compound is preformed in a rubber mold in applied pulse magnetic field of which high intensity introduces high degree of alignment of powder. Then the preform is compression-molded in said rubber mold in a static magnetic field. Rubber mold gives hydrostatic pressure which keeps the degree of alignment in the preform to be high. However the method has following drawbacks.
First, it is difficult to obtain high density because the applied pressure in compression molding must not be more than 0.8-1.5 ton/cm2 to protect the rubber mold from breakage. Insufficient density result in unsatisfactory magnetic properties. Second, precise dimension can not be obtained because of the deformation of the rubber mold under the applied pressure. Third, the method is not applicable to mass production due to wear of rubber mold.
The other patent that utilizes hydrostatic pressure is Japanese Patent Application Laid-Open (kokai) No. 61-147997. In the invention a cold isostatic press is applied in the compression molding with pulse or static magnetic field. The hydrostatic pressure in the process can avoid alignment deterioration and gives the magnet good magnetic properties. However, it is also not applicable to mass production because lifetime of the rubber mold is very short.
In our Japanese Patent Application Laid-Open (kokai) No. 8-31677, we have disclosed other method different from applying hydrostatic pressure to achieve high degree of alignment. In the invention the warm compression molding method is proposed to offer both precise dimension and high degree of alignment. In the process, metal dies instead of rubber mold is used to assure precise dimension. High degree of magnet powder alignment is obtained by applying both magnetic field and pressure on liquid state of resin in which the particles of magnet can align easily to the direction of applied magnetic field. The resin is heated to the temperature at which it is melted into liquid state just before the thermosetting reaction starts. However the achieved density has a certain limitation because it is hard to compress the compound at sufficiently high pressure due to following reason. In the invention the aligning and the densifying must be carried out simultaneously in one mold which is made of soft magnetic material. Soft magnetic material has low compression strength. Therefore sufficient pressure for obtaining high density compact can not be applied. Further this method has a drawback of low productivity, because of long press cycle time that consists of heating, loading, and magnetizing and demagnetizing the electromagnetic coil.